This section provides background information related to the present disclosure which is not necessarily prior art.
Photoconductive switches have been widely used in a wide variety of applications where incident radiation is used to trigger an electrical switch. One challenge that occurs with photoconductive switches is the electric field enhancement that occurs when a photoconductive substrate is rendered conducting by the application of incident radiation. If a portion of the substrate borders a non-conducting material which is adjacent to an electrode that is also in contact with the substrate, then when the substrate is rendered conducting the electric field will be expelled from the substrate that is in contact with the non-conducting material. This causes a large field enhancement to occur at an area known as the “Triple Point.” The Triple Point is the area where the conductor (i.e., electrode), the substrate supporting the conductor, and a non-conductor are in contact. This is illustrated in the simplified side cross-sectional views of FIGS. 1 and 2, where a portion of an electrode 1 resides on a substrate 2, and the electrode 1 and substrate 2 are interfaced with a dielectric 3. The Triple Point 4 is the area of interface of these three regions (i.e., the Contact Boundary dashed line extends through the Triple Point 4). FIG. 1 shows that the equipotential at the Triple Point is not substantially enhanced from its average value between the flat portion of the electrode, where the flat portion is denoted by length “L”. FIG. 2 shows the substrate 2 after it is has been rendered conducting. The equipotential lines 5 have been expelled from the portion of the substrate 2 which is in contact with the dielectric 3. This extreme distortion of the equipotentials around the Triple Point 4 results in a large field enhancement at this point that this not present when the substrate 2 is in the off state. It is believed that the reason for this is that when charge carriers are present, they move to the substrate-dielectric boundary to shield the interior of the substrate from an external electric field. The result is a charge accumulation of close to the Triple Point that generates a nearly singular electric field.
Careful design of the electrode shape can help to substantially reduce the electric field enhancement that occurs at the triple point when the substrate is non-conducting. However, once the substrate is activated, a large field enhancement still appears because carriers in the substrate, unable to move into the non-conducting material, accumulate at the interface. This causes a large field distortion at the triple point.
Previous design work involving switch packaging has considered electric field distributions in the state where the photoconductive substrate is non-conducting. It is under this condition that the voltage across the device will be at its maximum value. It has been found experimentally that when the substrates were illuminated to render them conductive, electrical breakdowns were found to occur at lower voltages than the package could withstand in the absence of illumination. A study of the electric field distribution in the case when the substrate is conducting revealed the presence of a new field enhancement due to carrier charge accumulation at the substrate boundary. This phenomenon appears to have been first noticed in the medical industry when electrodes were applied to the human body. Burns occurred on the skin at the edges of the electrodes due to this mechanism. Application of an additional conducting material between the electrode and the skin (that extended beyond the electrode) was found to modify the field distribution.